1. Field of the Invention
The present invention relates to a PAL (Phase Alternation by Line) encoder and more particularly to a PAL encoder in which a color signal processing is implemented by using a digital image signal processing.
2. Description of Related Art
FIG. 1 shows an example of an arrangement of a conventional PAL encoder. The PAL encoder shown in FIG. 1 is described in "Television", 1975, Vol. 29, No. 10 pp760-771, published from "Television Gakkai (Television Institute)".
The PAL encoder shown in FIG. 1 comprises a matrix circuit 101, low-pass filters (LPFs) 102, 103, balanced modulators 104, 105, an adder 106, a chrominance subcarrier generator 107, phase converters 109 to 112, a burst gate 113, and a switch circuit S101. Input terminals T101 to T103 for inputting color signals ER, EG, and EB are connected to the matrix circuit 101. The LPFs 102, 103 limit bands of a U-axis signal EU and V-axis signal EV from the matrix circuit 101. The balanced modulators 104 and 105 balance-modulate the outputs LEV, LEU from the LPFs 102 and 103 in accordance with the outputs from the phase converters 109 to 112. The adder 106 adds the outputs from the balanced modulators 104 and 105. The chrominance subcarrier generator 107 generates a chrominance subcarrier signal CS. The phase converters 109 to 112 are cascade connected to perform the phase conversions of -45 degrees, -90 degrees, -45 degrees, and -90 degrees, respectively.
An operation of the conventional PAL encoder shown in FIG. 1 will now be described.
The color signals ER, EG, and EB are supplied to the input terminals T101 to T103. The matrix circuit 101 converts the color signals ER, EG, and EB into U- and V-axes signals EU and EV in the UCS system of color representation, one of CIE standard colorimetric system. The high-frequency components of the U- and V-axes signals EU and EV are removed by the LPFs 102 and 103, resulting in signals LEV and LEU. Then, the signals LEV and LEU are input to the balanced modulators 104 and 105.
On the other hand, the chrominance subcarrier generator 107 generates a chrominance subcarrier signal CS=sin (.omega.t+270.degree.) and supplies it to the phase converters 109 to 112. Note that .omega.=2.pi. fsc wherein fsc represents a chrominance subcarrier frequency in the PAL system and t represents time. The phase converters 109 to 112 outputs signals sin (.omega.t+225.degree.), sin (.omega.t+135.degree.), sin (.omega.t+90.degree.) and sin (.omega.t). Accordingly, the signal sin (.omega.t) is input to the balanced modulator 105.
Further, the signals cos (.omega.t) (=sin (.omega.t+90.degree.) and -cos (.omega.t) (=sin (.omega.t+270.degree.) is selected by the switch circuit S101 controlled by a line switch signal LS input from the input terminal 105. The selected signal is input to the balanced modulator 104. The line switch signal LS becomes high (H) and low (L) levels in the odd and even lines of the first and second fields, respectively. Further, the signal LS becomes L and H levels in the odd and even lines of the third and fourth fields, respectively.
As a result, LEU sin (.omega.t) and LEV cos (.omega.t)/-LEV cos (.omega.t) are output from the balanced modulators 105 and 104, respectively. In this embodiment, a signal switched for every line is indicated by using a delimiter "/".
The switch circuit S102 controlled by the line switch signal LS selects and supplies to the burst gate 113 the signal sin (.omega.t+135.degree.) in the odd lines of the first and second fields and in the even lines of the third and fourth fields, and selects and supplies to the burst gate 113 the signal -sin (.omega.t+135.degree.) (=sin (.omega.t+225.degree.)) in the even lines of the first and second fields and in the odd lines of the third and fourth fields. The burst gate 113 performs a gain control for a burst level adjustment with respect to the supplied signal and passes the input signal only during a burst period.
The adder 106 adds the outputs of the balanced modulators 104 and 105 and the output of the burst gate 113, and outputs the added result from the output terminal T106 as the PAL color signal component.
The above described PAL encoder comprises an analog signal processing circuit. For this reason, it is difficult to suppress the phase distortion of the signal. For this reason, if a high-frequency characteristic in a signal processing is improved in order to implement a high resolution system, the frequency characteristic is affected by the phase distortion or the like, and thus it is difficult to keep the accuracy of the color signal in the PAL system. Further, when the PAL encoder is to be integrated by an LSI, many peripheral components are required, resulting in increased check items.